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          Python调用系统命令
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        <h2 id="os-system"><a href="#os-system" class="headerlink" title="os.system"></a>os.system</h2><p>该方法在调用完shell脚本后,返回一个16位的二进制数.<br>低位为杀死所调用脚本的信号号码,高位为脚本的退出状态码.</p>
<blockquote>
<p>脚本中”exit 1”的代码执行后, os.system函数返回值的高位数则是1;<br>如果低位数是0的情况下, 则函数的返回值是0x100, 换算为10进制得到256.</p>
</blockquote>
<p>如果我们需要获得os.system的正确返回值,那使用位移运算可以还原返回值:</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">&gt;&gt;&gt; </span>n = os.system(test.sh)</span><br><span class="line"><span class="meta">&gt;&gt;&gt; </span>n &gt;&gt; <span class="number">8</span></span><br><span class="line"><span class="meta">&gt;&gt;&gt; </span><span class="number">3</span></span><br></pre></td></tr></table></figure>

<p>这是最简单的一种方法, 特点是执行的时候程序会打出cmd在linux上执行的信息.<br>使用前需要import os.</p>
<blockquote>
<p>os.system(“ls”)  仅仅在一个子终端运行系统命令, 而不能获取命令执行后的返回信息</p>
</blockquote>
<a id="more"></a>

<h2 id="os-Popen"><a href="#os-Popen" class="headerlink" title="os.Popen"></a>os.Popen</h2><p>这种调用方式是通过管道的方式来实现,函数返回一个file-like的对象,里面的内容是脚本输出的内容(可简单理解为echo输出的内容).</p>
<p>python调用Shell脚本,有两种方法:</p>
<ul>
<li>os.system(cmd)</li>
<li>os.popen(cmd),</li>
</ul>
<p>前者返回值是脚本的退出状态码, 后者的返回值是脚本执行过程中的输出内容. 实际使用时视需求情况而选择。<br>明显地,像调用”ls”这样的shell命令,应该使用popen的方法来获得内容</p>
<blockquote>
<p>popen(command [, mode=’r’ [, bufsize]]) -&gt; pipe</p>
</blockquote>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">tmp = os.popen(<span class="string">'ls *.py'</span>).readlines()</span><br></pre></td></tr></table></figure>

<h2 id="subprocess-Popen"><a href="#subprocess-Popen" class="headerlink" title="subprocess.Popen"></a>subprocess.Popen</h2><p>现在大部分人都喜欢使用Popen。Popen方法不会打印出cmd在linux上执行的信息。的确，Popen非常强大，支持多种参数和模式。使用前需要:</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">from</span> subprocess <span class="keyword">import</span> Popen, PIPE</span><br></pre></td></tr></table></figure>

<p>但是Popen函数有一个缺陷，就是它是一个阻塞的方法。如果运行cmd时产生的内容非常多，函数非常容易阻塞住。解决办法是不使用wait()方法，但是也不能获得执行的返回值了。</p>
<p>Popen原型是：</p>
<blockquote>
<p>subprocess.Popen(args, bufsize=0, executable=None, stdin=None, stdout=None, stderr=None, preexec_fn=None, close_fds=False, shell=false)</p>
</blockquote>
<ul>
<li>参数bufsize：指定缓冲。我到现在还不清楚这个参数的具体含义，望各个大牛指点。</li>
<li>参数executable用于指定可执行程序。一般情况下我们通过args参数来设置所要运行的程序。如果将参数shell设为 True，executable将指定程序使用的shell。在windows平台下，默认的shell由COMSPEC环境变量来指定。</li>
<li>参数stdin, stdout, stderr分别表示程序的标准输入、输出、错误句柄。他们可以是PIPE，文件描述符或文件对象，也可以设置为None，表示从父进程继承。</li>
<li>参数preexec_fn只在Unix平台下有效，用于指定一个可执行对象（callable object），它将在子进程运行之前被调用。</li>
<li>参数Close_sfs：在windows平台下，如果close_fds被设置为True，则新创建的子进程将不会继承父进程的输入、输出、错误管道。我们不能将close_fds设置为True同时重定向子进程的标准输入、输出与错误(stdin, stdout, stderr)。</li>
<li>如果参数shell设为true，程序将通过shell来执行。</li>
<li>参数cwd用于设置子进程的当前目录。</li>
<li>参数env是字典类型，用于指定子进程的环境变量。如果env = None，子进程的环境变量将从父进程中继承。</li>
<li>参数Universal_newlines:不同操作系统下，文本的换行符是不一样的。如：windows下用’/r/n’表示换，而Linux下用 ‘/n’。如果将此参数设置为True，Python统一把这些换行符当作’/n’来处理。</li>
<li>参数startupinfo与createionflags只在windows下用效，它们将被传递给底层的CreateProcess()函数，用于设置子进程的一些属性，如：主窗口的外观，进程的优先级等等。</li>
<li>subprocess.PIPE, 在创建Popen对象时，subprocess.PIPE可以初始化stdin, stdout或stderr参数，表示与子进程通信的标准流。</li>
<li>subprocess.STDOUT, 创建Popen对象时，用于初始化stderr参数，表示将错误通过标准输出流输出。</li>
</ul>
<p>Popen的方法：</p>
<ul>
<li>Popen.poll()<br>  用于检查子进程是否已经结束。设置并返回returncode属性。</li>
<li>Popen.wait()<br>  等待子进程结束。设置并返回returncode属性。</li>
<li>Popen.communicate(input=None)<br>  与子进程进行交互。向stdin发送数据，或从stdout和stderr中读取数据。可选参数input指定发送到子进程的参数。 Communicate()返回一个元组：(stdoutdata, stderrdata)。注意：如果希望通过进程的stdin向其发送数据，在创建Popen对象的时候，参数stdin必须被设置为PIPE。同样，如 果希望从stdout和stderr获取数据，必须将stdout和stderr设置为PIPE。</li>
<li>Popen.send_signal(signal)<br>  向子进程发送信号。</li>
<li>Popen.terminate()<br>  停止(stop)子进程。在windows平台下，该方法将调用Windows API TerminateProcess（）来结束子进程。</li>
<li>Popen.kill()<br>  杀死子进程。</li>
<li>Popen.stdin<br>  如果在创建Popen对象是，参数stdin被设置为PIPE，Popen.stdin将返回一个文件对象用于策子进程发送指令。否则返回None。</li>
<li>Popen.stdout<br>  如果在创建Popen对象是，参数stdout被设置为PIPE，Popen.stdout将返回一个文件对象用于策子进程发送指令。否则返回 None。</li>
<li>Popen.stderr<br>  如果在创建Popen对象是，参数stdout被设置为PIPE，Popen.stdout将返回一个文件对象用于策子进程发送指令。否则返回 None。</li>
<li>Popen.pid<br>  获取子进程的进程ID。</li>
<li>Popen.returncode<br>  获取进程的返回值。如果进程还没有结束，返回None。</li>
</ul>

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